Renal Pharmacology

Correct Answer

B — Calcium gluconate

Rationale

Calcium gluconate is the first-line membrane stabilizer in severe hyperkalemia with electrocardiogram changes. It raises the threshold potential of cardiac myocytes, widening the gap between resting membrane potential and threshold and reducing the cardiac excitability that produces peaked T waves, widened QRS complexes, and arrhythmia. Patiromer is a gastrointestinal potassium binder used for elimination. Furosemide is a loop diuretic that promotes renal potassium excretion. Insulin redistributes potassium intracellularly.

Correct Answer

D — Insulin

Rationale

Insulin drives potassium into skeletal muscle cells by stimulating sodium-potassium-ATPase activity, shifting approximately 0.5 to 1.5 milliequivalents per liter of serum potassium intracellularly within 15 to 30 minutes. It is classified as a potassium redistribution agent — it temporarily lowers serum potassium without removing it from the body. Calcium gluconate is a membrane stabilizer. Patiromer is a gastrointestinal potassium binder that eliminates potassium from the body. Sodium bicarbonate also redistributes potassium but through hydrogen-potassium exchange rather than sodium-potassium-ATPase stimulation.

Correct Answer

A — Patiromer

Rationale

Patiromer is a gastrointestinal cation exchanger that binds potassium in the distal colon and removes it in the stool, making it a potassium elimination agent rather than a redistribution agent. Sodium zirconium cyclosilicate is a similar cation exchanger that acts throughout the gastrointestinal tract. Furosemide promotes renal potassium excretion as a loop diuretic. Insulin and calcium gluconate do not remove potassium from the body — they redistribute or stabilize cardiac membranes, respectively.

Correct Answer

C — Sodium bicarbonate

Rationale

Sodium bicarbonate is an alkali therapy that directly replaces or supplements serum bicarbonate, correcting metabolic acidosis. In hyperkalemia management, it shifts potassium intracellularly through hydrogen-potassium exchange across the cell membrane — as extracellular pH rises, hydrogen ions move out of cells and potassium moves in to maintain electrical neutrality. This redistributive effect is most pronounced when concurrent metabolic acidosis is present. Patiromer eliminates potassium via the gut. Calcium gluconate stabilizes cardiac membranes. Furosemide promotes renal potassium excretion.

Correct Answer

B — Amphotericin B

Rationale

Amphotericin B is a polyene antifungal that inserts into ergosterol-rich fungal membranes. At renal tubular concentrations it forms similar pores in tubular cell membranes, producing distal tubular acidification defects, renal potassium wasting, and renal magnesium wasting. Voriconazole is a triazole antifungal without this tubular toxicity profile. Tacrolimus is a calcineurin inhibitor causing afferent arteriolar nephrotoxicity. Canagliflozin is a sodium-glucose cotransporter 2 inhibitor acting in the proximal tubule.

Correct Answer

D — Desmopressin

Rationale

Desmopressin is a synthetic vasopressin analog that activates vasopressin type 2 receptors in the collecting duct, promoting aquaporin-2 insertion and reducing free water excretion. When sodium has risen faster than the safe correction limit of 10 to 12 milliequivalents per liter per 24 hours, desmopressin is given alongside hypotonic intravenous fluids to slow the rate of sodium rise and prevent osmotic demyelination syndrome. Tolvaptan is a vasopressin type 2 receptor antagonist — the pharmacological opposite — used to raise sodium, not slow its correction. Furosemide and patiromer do not affect sodium correction rate.

Correct Answer

A — Calcium raises the threshold potential of cardiac myocytes, widening the gap between resting membrane potential and the threshold for depolarization and reducing excitability

Rationale

Hyperkalemia depolarizes the resting membrane potential of cardiac myocytes by reducing the electrochemical gradient for potassium across the membrane, bringing the resting potential closer to the threshold for spontaneous depolarization and producing the characteristic electrocardiogram changes. Calcium gluconate raises the threshold potential — moving it further from the resting potential — thereby restoring a wider safety margin and reducing cardiac excitability. This effect begins within minutes and lasts 30 to 60 minutes. Calcium does not lower serum potassium; it buys time for redistribution and elimination measures to work.

Correct Answer

C — Albuterol activates beta-2 adrenergic receptors on skeletal muscle, stimulating cyclic adenosine monophosphate-mediated sodium-potassium-ATPase activation and driving potassium intracellularly

Rationale

Albuterol at four to eight times the standard bronchodilator dose (10 to 20 mg nebulized) activates beta-2 adrenergic receptors on skeletal muscle cells. This raises intracellular cyclic adenosine monophosphate, which activates sodium-potassium-ATPase, driving potassium from the extracellular space into muscle cells. The result is a reduction in serum potassium of approximately 0.5 to 1.0 milliequivalents per liter, additive to the effect of insulin. Albuterol is a redistribution agent — like insulin, it does not remove potassium from the body. Tachycardia limits its use in patients with ischemic heart disease.

Correct Answer

D — Rapid infusion of hypertonic saline directly raises serum sodium concentration, increasing plasma osmolality and reducing cerebral edema to abort active seizures

Rationale

In acute severe hyponatremia with neurological symptoms such as seizures, the immediate priority is raising serum sodium rapidly enough to reduce cerebral edema and abort the seizure. A bolus of 100 to 150 milliliters of 3% sodium chloride, which contains 513 milliequivalents per liter of sodium, raises serum sodium by approximately 2 to 3 milliequivalents per liter within minutes — sufficient to reduce brain swelling and stop active seizures. The high sodium concentration directly raises serum osmolality, drawing water from cerebral cells into the vascular compartment. Once the acute seizure is controlled, the rate of correction is slowed to remain within the safe limit of 10 to 12 milliequivalents per liter per 24 hours.

Correct Answer

B — The brain adapted to chronic hyponatremia by extruding intracellular osmoles; rapid sodium correction creates an osmolality mismatch that draws water out of myelin-rich neurons, disrupting myelin sheaths

Rationale

In chronic hyponatremia, the brain protects cell volume by extruding intracellular osmoles — organic solutes that take days to regenerate. When serum sodium rises rapidly, the extracellular osmolality increases faster than the brain cells can restore their intracellular osmole content. Water moves out of these osmole-depleted neurons down the osmolality gradient, shrinking myelin-rich cells and disrupting myelin sheaths in the pons and extrapontine regions. The resulting osmotic demyelination syndrome causes the neurological deficits described and is largely irreversible. Patients at highest risk are those with severe, chronic hyponatremia — particularly those with alcoholism, malnutrition, or liver disease.

Correct Answer

A — The reaction of bicarbonate with hydrogen ions generates carbon dioxide, which freely crosses the blood-brain barrier and lowers intracellular and cerebrospinal fluid pH even as extracellular pH rises

Rationale

When sodium bicarbonate buffers extracellular hydrogen ions, the reaction produces carbon dioxide and water. Carbon dioxide is lipid-soluble and freely diffuses across the blood-brain barrier and into cells, where it is hydrated by carbonic anhydrase to generate carbonic acid, lowering intracellular and cerebrospinal fluid pH. This can paradoxically worsen intracellular acidosis in the brain and heart even as arterial pH improves. Additionally, the newly generated carbon dioxide may blunt the hyperventilatory drive that was partially compensating for the metabolic acidosis. These limitations restrict routine sodium bicarbonate use in lactic acidosis and diabetic ketoacidosis to situations of extreme acidemia below a pH of 6.9.

Correct Answer

C — In this volume-overloaded patient the alkalosis is maintained by ongoing mineralocorticoid activity and loop diuretic use, not by volume depletion; acetazolamide directly promotes bicarbonate excretion without requiring saline volume repletion

Rationale

This patient has a high urine chloride, indicating ongoing mineralocorticoid-driven chloride-resistant metabolic alkalosis from loop diuretic use and secondary aldosteronism — not chloride-responsive alkalosis from volume depletion. In chloride-responsive alkalosis (urine chloride below 20 milliequivalents per liter), the treatment is isotonic saline to restore volume and chloride, which allows the kidney to excrete excess bicarbonate. In this volume-overloaded patient who cannot receive saline, acetazolamide is appropriate because it directly inhibits proximal tubular carbonic anhydrase, impairing bicarbonate reabsorption and promoting bicarbonate excretion without adding volume. This targeted approach corrects the alkalosis and may improve the response to continued loop diuretic therapy.

Correct Answer

D — Amphotericin B forms pores in tubular cell membranes at renal concentrations, disrupting the electrochemical gradients that drive potassium and magnesium reabsorption in the distal nephron

Rationale

Amphotericin B inserts into cell membranes containing ergosterol and forms ion-permeable pores. At the concentrations achieved in renal tubular cells, it forms similar pores in tubular membranes, disrupting the electrochemical gradients and ion selectivity required for potassium and magnesium reabsorption in the distal nephron. The result is a distal tubular acidification defect resembling type 1 renal tubular acidosis, along with renal potassium and magnesium wasting. Liposomal amphotericin B encapsulates the drug in lipid vesicles that reduce free drug delivery to renal tubular cells while maintaining delivery to fungal pathogens, substantially reducing nephrotoxicity.

Correct Answer

B — Tenofovir accumulates in proximal tubule mitochondria via organic anion transporters and inhibits mitochondrial deoxyribonucleic acid polymerase gamma, causing energy failure that disrupts all energy-dependent proximal tubular reabsorption

Rationale

Tenofovir disoproxil fumarate is actively secreted into proximal tubular cells by organic anion transporters, where it accumulates in mitochondria and inhibits mitochondrial deoxyribonucleic acid polymerase gamma. This impairs mitochondrial energy production in tubular cells that depend heavily on oxidative metabolism. The resulting energy failure disrupts all energy-dependent reabsorptive transporters at the proximal convoluted tubule simultaneously — sodium-coupled glucose transport, phosphate transport, amino acid transport, and bicarbonate reclamation — producing the Fanconi syndrome pattern of glycosuria with normal blood glucose, hypophosphatemia, aminoaciduria, and proximal renal tubular acidosis. Persistent phosphate wasting from this mechanism causes osteomalacia.

Correct Answer

C — Calcium gluconate raised the threshold potential of cardiac myocytes, restoring cardiac electrical stability without affecting serum potassium concentration

Rationale

The electrocardiogram normalization without any change in serum potassium confirms that calcium gluconate acts purely as a membrane stabilizer. By raising the threshold potential of cardiac myocytes, it widens the gap between the depolarized resting membrane potential (caused by hyperkalemia) and the threshold for spontaneous depolarization, restoring the normal safety margin for cardiac conduction. This protective effect begins within minutes and lasts 30 to 60 minutes. Because the serum potassium remains dangerously elevated, redistribution with insulin and dextrose and elimination by hemodialysis must be initiated promptly — calcium gluconate has only bought time.

Correct Answer

A — Magnesium depletion prevents adequate suppression of renal outer medullary potassium channel-mediated potassium secretion in the collecting duct, causing ongoing renal potassium wasting despite replacement

Rationale

Magnesium normally suppresses renal outer medullary potassium channel activity in the collecting duct, limiting potassium secretion. When cisplatin damages the TRPM6 magnesium channel in the distal convoluted tubule and causes magnesium wasting, the resulting magnesium depletion removes this suppressive effect on potassium secretion. The collecting duct continues to secrete potassium at an elevated rate regardless of how much potassium is replaced, making hypokalemia refractory. Concurrent intravenous magnesium repletion is required before serum potassium will normalize — a principle that applies to hypomagnesemia from any cause, not only cisplatin.

Correct Answer

D — Administer desmopressin to reduce free water excretion and infuse 5% dextrose in water to actively lower serum sodium back toward the target range

Rationale

When serum sodium has risen faster than 10 to 12 milliequivalents per liter per 24 hours, the sodium must be actively re-lowered to prevent osmotic demyelination syndrome. Desmopressin activates vasopressin type 2 receptors in the collecting duct, promoting aquaporin-2 insertion and reducing free water excretion, which slows the ongoing rise in serum sodium. Concurrently, intravenous 5% dextrose in water provides free water to dilute the elevated serum sodium back toward the target range. The goal is to keep the cumulative 24-hour sodium correction below 10 to 12 milliequivalents per liter. Continuing to raise sodium or promoting further water excretion would increase the risk of irreversible neurological injury.

Correct Answer

B — The low urine chloride identifies this as chloride-responsive alkalosis maintained by volume and chloride depletion; isotonic saline restores chloride and volume, allowing the kidney to excrete the excess bicarbonate

Rationale

A urine chloride below 20 milliequivalents per liter indicates chloride-responsive metabolic alkalosis — the kidney is avidly retaining sodium and chloride because of volume and chloride depletion from nasogastric suctioning of hydrochloric acid. The alkalosis is maintained because the volume-depleted kidney reabsorbs bicarbonate to preserve volume in the absence of adequate chloride. Isotonic saline provides chloride and restores intravascular volume, removing the stimulus for bicarbonate retention and allowing the kidney to excrete the excess bicarbonate. Acetazolamide is appropriate in chloride-resistant alkalosis from ongoing mineralocorticoid excess in volume-overloaded patients who cannot receive saline — the opposite clinical scenario from this patient.